It is known that image compression such as JPEG (Joint Photographic Experts Group) using spatial frequency conversion such as DCT (Discrete Cosine Transform) results in low image quality due to mosquito noise in an edge part of a character or the like. In particular, if an image is printed on a print paper sheet that has a white background, such low image quality is emphasized because toner is put on such a part that should be white as original of the background.
An image forming apparatus separates only a character part and performs gamma conversion for the separated character part and thereby removes its low density part, and consequently removes mosquito noise (see PATENT LITERATURE #1).